JP2007025369A - Optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical modulator which works at a high speed, has a small alpha parameter and a large extinction ratio, and also has a small driving voltage and DC bias voltage. <P>SOLUTION: The optical modulator has a Mach-Zehnder optical waveguide equipped with a substrate 1 having electrooptic effect, an optical waveguide 12 formed on the substrate 1 and guiding light, and a progressive-wave electrode 4 comprising a center conductor 4a for a high-frequency electric signal formed on one surface side of the substrate 1 and applying the high-frequency electric signal for modulating light and ground conductors 4b and 4c, and the optical waveguide 12 has a plurality of optical waveguides 12a and 12b for interaction which modulate the phase of the light by applying the high-frequency electric signal to the progressive-wave electrode 4. The plurality of optical waveguides 12a and 12b for interaction have at least one or more high-refractive-index areas 13a and 13b, where the plurality of optical waveguides 12a and 12b for interaction are different in refractive index from each other in a section perpendicular to the length of the optical waveguide 12, partially along the lengths. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention belongs to the field of optical modulators having high speed, a small alpha parameter, a large extinction ratio, and a small driving voltage and DC bias voltage.

An optical waveguide and a traveling wave electrode are provided on a substrate having a so-called electro-optic effect (hereinafter, the lithium niobate substrate is abbreviated as an LN substrate) such as lithium niobate (LiNbO ₃ ) whose refractive index is changed by applying an electric field. The formed traveling wave electrode type lithium niobate optical modulator (hereinafter abbreviated as LN optical modulator) is applied to a 2.5 Gbit / s, 10 Gbit / s large capacity optical transmission system because of its excellent chirping characteristics. Yes. Recently, application to an ultra large capacity optical transmission system of 40 Gbit / s is also being studied, and it is expected as a key device.

[First prior art]
This LN optical modulator includes a type using a z-cut substrate and a type using an x-cut substrate (or y-cut substrate). Here, an x-cut substrate LN optical modulator using an x-cut LN substrate and a coplanar waveguide (CPW) traveling wave electrode is taken up as a first prior art, and a perspective view thereof is shown in FIG. FIG. 9 is a cross-sectional view taken along line AA ′ of FIG. The following discussion holds true for z-cut substrates as well.

In the figure, 1 is an x-cut LN substrate, 2 is a transparent SiO ₂ buffer layer having a thickness of about 200 nm to 1 μm in the wavelength region used in optical communication such as 1.3 μm or 1.55 μm, and 3 is an x-cut LN. This is an optical waveguide formed by thermally diffusing Ti at 1050 ° C. for about 10 hours after depositing Ti on the substrate 1, and constitutes a Mach-Zehnder interference system (or Mach-Zehnder optical waveguide). Reference numerals 3a and 3b denote optical waveguides (or interactive optical waveguides) in a portion where an electrical signal and light interact (referred to as an interaction portion), that is, two arms of a Mach-Zehnder optical waveguide. The CPW traveling wave electrode 4 includes a central conductor 4a and ground conductors 4b and 4c.

In FIG. 9, W _a and W _b are the widths of the interaction optical waveguides 3a and 3b. In the first prior art, the widths of the two interaction optical waveguides 3a and 3b are equal (that is, W _a = W _b , for example, W _a and W _b are both 9 μm). G _wg is a distance (also referred to as a waveguide gap) between the interactive optical waveguides 3a and 3b, and is, for example, 16 μm. Further, S _wg is the distance between the centers of the interactive optical waveguides 3a and 3b (in this example, it is 25 μm). Δ is the distance between the edge of the center conductor 4a and the center of the interaction optical waveguide 3a or 3b (in this figure, the distance between the edge of the center conductor 4a and the center of the interaction optical waveguide 3b).

  FIG. 10 shows a top view of only the optical waveguide 3. Here, let L be the length of the interaction optical waveguides 3a and 3b. Although FIG. 10 shows only the optical waveguide, A-A ′ is indicated at a position corresponding to A-A ′ in the perspective view of FIG. 8.

In the first prior art, a bias voltage (usually a DC bias voltage) and a high-frequency electric signal (also referred to as an RF electric signal) are superimposed and applied between the center conductor 4a and the ground conductors 4b and 4c. In the waveguide, not only the RF electrical signal but also the DC bias voltage changes the phase of the light. Further, the microwave effective index n _m of the interaction optical waveguides 3a of the buffer layer 2 is an electrical signal, by approximating the effective refractive index n _o of the light propagating the 3b, and important function of expanding the optical modulation band is doing.

  Next, the operation of the LN optical modulator configured as described above will be described. In order to operate this LN optical modulator, it is necessary to apply a DC bias voltage and an RF electrical signal between the center conductor 4a and the ground conductors 4b and 4c.

  The voltage-light output characteristics shown in FIG. 11 are the voltage-light output characteristics of the LN optical modulator in a certain state, and Vb is the DC bias voltage at that time. As shown in FIG. 11, the DC bias voltage Vb is normally set at the midpoint between the peak and bottom of the light output characteristic.

FIG. 12 shows the product of the half-wave voltage Vπ and the length L of the interactive optical waveguide (referred to as Vπ · L, which is a measure for considering the driving voltage), the edge of the center conductor 4a, and the interactive optical waveguide. The relationship with the distance Δ from the center of 3b is shown. In this calculation, the value of Δ is determined by changing the gap _Gwg between the optical waveguides 3a and 3b. From FIG. 12, it can be seen that the distance Δ between the edge of the center conductor 4a and the center of the interactive optical waveguide 3b should be small to some extent, and further, there exists an optimum value.

Therefore, if the distance Δ between the edge of the center conductor 4a and the center of the interaction optical waveguide 3b (and 3a) is decreased in order to reduce the drive voltage, the gap _Gwg between the interaction optical waveguides 3a and 3b is increased. Get smaller. However, as shown in FIG. 13, when the gap _Gwg between the interaction optical waveguides 3a and 3b decreases, the degree of coupling between the interaction optical waveguides 3a and 3b increases remarkably, and the power when the light is turned on / off is increased. There is a problem that deterioration of the ratio, that is, extinction ratio occurs.

[Second prior art]
In general, in order to reduce the degree of coupling between two optical waveguides, the distance _Gwg between the two optical waveguides is increased to keep the light propagating through each optical waveguide away from each other, or the two optical waveguides There is a method of suppressing mutual coupling by changing the widths of the waveguides and changing the equivalent refractive index (or propagation constant) of light propagating through the two optical waveguides.

However, when the distance _Gwg between the two optical waveguides is increased, Vπ · L is increased as described in FIG. 12, and as a result, a high drive voltage is required. Therefore, in the second prior art shown as a cross-sectional view in FIG. 14, the widths W _a ′ and W _b ′ of the two interactive optical waveguides 5a and 5b are made different. FIG. 15 shows a top view of only the optical waveguide 5 of the second prior art. In FIG. 14, the x-cut LN substrate 1, the central conductor 4a, the ground conductors 4b and 4c, and the buffer layer 2 are included in the cross-sectional view taken along the line BB ′ of FIG.

However, this second prior art has several problems. First, it is considered that the widths of the interaction optical waveguides 5a and 5b are made different while the distance _Swg ′ between the centers of the interaction optical waveguides 5a and 5b is kept constant. For example, the width W _a ′ of the interaction optical waveguide 5a is narrowed, and the width W _b ′ of the interaction optical waveguide 5b is increased. In this case, since the width W _a ′ of the interaction optical waveguide 5a is narrowed, the light propagating through the interaction optical waveguide 5a leaks out of the interaction optical waveguide 5a, and as a result, the spot size increases. Further, since the width W _b ′ of the interaction optical waveguide 5b is increased, the spot size of light propagating through the interaction optical waveguide 5a is also increased. Since the increase in the spot size of light propagating through the interaction optical waveguides 5a and 5b corresponds to the reduction in the distance _Gwg 'between the interaction optical waveguides 5a and 5b, A part of the effect of suppressing the coupling by changing the equivalent refractive index of the interaction optical waveguides 5a and 5b is canceled out, so that a sufficient effect cannot be obtained.

On the other hand, when the width W _b ′ of the interaction optical waveguide 5b is widened while keeping the distance between the edge of the center conductor 4a and the edge of the interaction optical waveguide 5b constant in FIG. 14, the edge of the center conductor 4a and the interaction light are increased. The distance Δ at the center of the waveguide 5b is increased, and Vπ · L, which is a measure for evaluating the drive voltage, is increased as shown in FIG.

Further, as shown in FIG. 15, in the second prior art, the widths W _a ′ and W _b ′ of the interaction optical waveguides 3a and 3b are partially changed, and 6, 7 and 7 in FIG. Reference numerals 8, 9, 10, and 11 denote the tapered regions whose width changes. As is well known, when the width of the optical waveguide is changed in this way, radiation loss occurs in that region. Therefore, the powers of light propagating through the interaction optical waveguides 3a and 3b are different from each other, and the extinction ratio as an optical modulator is deteriorated.

  As described above, when the two interactive optical waveguides are brought close to the central conductor as in the first prior art in order to reduce the drive voltage, the two interactive optical waveguides approach each other. As a result, light is combined, resulting in deterioration of the extinction ratio. Further, as in the second prior art, if the widths of the two interaction optical waveguides are made different, this corresponds to the fact that the distance between the two interaction optical waveguides becomes narrower. A part of the effect of suppressing the coupling by changing the equivalent refractive index of the interactive optical waveguides 5a and 5b is canceled out, so that the sufficient effect cannot be obtained, the drive voltage is increased, or the optical modulator There was a problem that the extinction ratio deteriorated.

  In order to solve the above problems, an optical modulator according to claim 1 of the present invention includes a substrate having an electro-optic effect, an optical waveguide for guiding light formed on the substrate, and one of the substrates. A traveling wave electrode formed on the surface side and made up of a center conductor and a ground conductor for a high frequency electrical signal for applying a high frequency electrical signal for modulating the light; Comprising a Mach-Zehnder optical waveguide having a plurality of interacting optical waveguides for modulating the phase of the light by applying the high-frequency electric signal, each of the plurality of interacting optical waveguides being in the longitudinal direction of the optical waveguide At least one region having different refractive indexes of the plurality of interactive optical waveguides in a cross section perpendicular to the longitudinal direction.

  The optical modulator according to claim 2 of the present invention is the optical modulator according to claim 1, wherein the magnitude relationship of the refractive index in the longitudinal direction is switched between the mutually interacting optical waveguides. A region is provided in each of the interacting optical waveguides.

  An optical modulator according to a third aspect of the present invention is the optical modulator according to the second aspect, wherein the two optical elements facing each other are arranged so that chirping of the light modulated by the high-frequency electrical signal becomes substantially zero. The relationship between the refractive indexes of the interacting optical waveguides is such that the length of the region is changed in the longitudinal direction of the interacting optical waveguide.

An optical modulator according to a fourth aspect of the present invention is the optical modulator according to any one of the second to third aspects, wherein the magnitude relationship of the refractive indices of the two interacting optical waveguides facing each other is the length of the interactive optical waveguide. In the direction, the lengths of L ₁ and L ₂ are interchanged, and L ₁ and L ₂ satisfy the relationship of L ₁ <L ₂ .

An optical modulator according to a fifth aspect of the present invention is the optical modulator according to any one of the second to third aspects, wherein a refractive index magnitude relationship between the two interacting optical waveguides facing each other is a length of the interactive optical waveguide. replaced with a length of _{L 1,} L 2, _{L 3} in the direction, said _{L 1,} said _{L 2,} wherein _{L 3} is characterized in that substantially satisfy a relation of _{L 1 = L 3 = L 2} /2.

  An optical modulator according to a sixth aspect of the present invention is the optical modulator according to any one of the second to fourth aspects, wherein the optical modulator is emitted from the interactive optical waveguide within a predetermined modulation frequency band based on the high-frequency electrical signal. The refractive index in the two interacting optical waveguides facing each other so that the sign of the alpha parameter representing the chirping magnitude of the optical signal pulse changes from plus to minus or from minus to plus as the modulation frequency changes. The size relationship is set to the length of the region where the optical waveguide is switched in the longitudinal direction of the optical waveguide.

  An optical modulator according to a seventh aspect of the present invention is the optical modulator according to any one of the second to fourth aspects, wherein an integration value for the frequency of the alpha parameter in a predetermined frequency band based on the high-frequency electrical signal is substantially zero. As described above, the length of the region where the magnitude relationship of the refractive index of the two interacting optical waveguides facing each other is changed in the longitudinal direction of the interactive optical waveguide is set.

  According to the first aspect of the present invention, since the optical coupling between the two interactive optical waveguides can be suppressed by changing the refractive indexes of the two interactive optical waveguides constituting the Mach-Zehnder, the extinction ratio can be improved.

  According to a second aspect of the present invention, in the optical modulator according to the first aspect, the extinction ratio is improved by switching the magnitude relationship between the refractive indexes of the two interacting optical waveguides facing each other in the longitudinal direction of the optical waveguide. In addition to reducing chirping.

  According to a third aspect of the present invention, in the optical modulator according to the second aspect, the length of the region where the refractive index magnitude relationship between the two interacting optical waveguides facing each other interchanges in the longitudinal direction of the optical waveguide is appropriately set. As a result, the extinction ratio can be improved and chirping can be made almost zero.

According to a fourth aspect of the present invention, in the optical modulator according to the second to third aspects, if the length of the region where the magnitude relationship of the refractive index interchanges in the longitudinal direction of the optical waveguide is L ₁ and L ₂ , Considering the propagation loss of the high-frequency electrical signal propagating through the traveling wave electrode composed of the ground conductor, the extinction ratio can be improved and the chirping can be reduced or made zero by setting L ₁ <L ₂ .

According to a fifth aspect of the present invention, in the optical modulator according to the second to third aspects, the magnitude relationship between the refractive indexes of the two optical waveguides facing each other is L ₁ , L ₂ , L in the longitudinal direction of the optical waveguide. replaced with a length of _3, by their length _{L 1,} L 2, _{L 3} satisfy the relationship of _{L 1 = L 3 = L 2} /2, small chirping is possible improve the extinction ratio, or almost Can be zero.

  According to a sixth aspect of the present invention, in the optical modulator according to the second to fourth aspects, by appropriately setting the length of the region where the magnitude relationship of the refractive index is switched in the longitudinal direction of the optical waveguide, Since the sign of the alpha parameter indicating the magnitude of chirping changes from plus to minus or from minus to plus as the modulation frequency changes, extremely small chirping characteristics can be realized.

  According to a seventh aspect of the present invention, in the optical modulator according to the second to fourth aspects, by setting the length of the region where the magnitude relationship of the refractive index is switched in the longitudinal direction of the interactive optical waveguide, the frequency of the alpha parameter is set. Since the integral value for is almost zero, extremely small chirping characteristics can be realized.

  Hereinafter, embodiments of the present invention will be described. Since the same numbers as those in the prior art shown in FIGS. 8 to 15 correspond to the same function units, description of the function units having the same numbers is omitted here.

[First Embodiment]
FIG. 1 shows a cross-sectional view of a first embodiment of the present invention. FIG. 2 shows a top view of the optical waveguide 12. Here, 12a and 12b are interactive optical waveguides, 13a and 13b are high refractive index regions provided in the interactive optical waveguides 12a and 12b, and W _a ″ and W _b ″ include high refractive index regions 13a and 13b. The widths of the interaction optical waveguides 12a and 12b are assumed to be equal (that is, W _a ″ = W _b ″, for example, W _a ″ and W _b ″ are both 9 μm).

G _wg ″ is a distance (waveguide gap) between the interactive optical waveguides 12a and 12b including the high refractive index regions 13a and 13b, and is, for example, 16 μm. S _wg ″ is the distance between the centers of the interactive optical waveguides 12a and 12b including the high refractive index regions 13a and 13b (in this example, it is 25 μm). Δ is the distance between the edge of the central conductor 4a and the center of the interactive optical waveguide 12a including the high refractive index regions 13a and 13b or 12b (in this figure, the edge of the central conductor 4a and the center of the interactive optical waveguide 12b) ).

2, the interaction optical waveguides 12a, 12b in the region of the length L ₁ (hereinafter, the first region hereinafter) refraction of the refractive index of the interaction optical waveguide 12b of the high refractive index region 13a in the interaction optical waveguides 12a higher than the rate, the area of the length L ₂ (hereinafter, the second region is referred to as a) the refractive index of the high refractive index region 13b in the interaction optical waveguides 12b is higher than the refractive index of the interaction optical waveguides 12a. This state is shown in FIG. 3 as a refractive index distribution corresponding to CC ′ in FIGS.

  This structure is manufactured by the following procedure. First, after forming a resist pattern in the shape of the normal Mach-Zehnder optical waveguide 12 in FIG. 2, metal Ti is vapor-deposited. Further, after a resist pattern is formed in the shape of the regions 13a and 13b, Ti is additionally deposited. And high refractive index area | regions 13a and 13b can be formed in the interaction optical waveguides 12a and 12b by thermally diffusing both at once.

Here, in the first region of the length L _1, and the refractive index difference between the high refractive index region 13a and the interaction optical waveguide 12b and [Delta] n, the interaction optical waveguides 12a and interaction optical guide comprising a high refractive index region 13a FIG. 4 shows the relationship between the degree of coupling of light propagating through the waveguide 12b and the absolute value | Δn | (here, | Δn | = Δn) of the refractive index difference. As can be seen from the figure, when the refractive index difference Δn is increased, the degree of coupling of light propagating through the two interactive optical waveguides 12a and 12b including the high refractive index regions 13a and 13b is significantly reduced. Also in the second region of length L _2, and the refractive index difference between the high refractive index region 13b and the interaction optical waveguides 12a and [Delta] n, the interaction optical waveguide 12b and the interaction optical guide comprising a high refractive index region 13b The relationship between the degree of coupling of light propagating through the waveguide 12a and the absolute value | Δn | of the refractive index difference is also given in the same manner as in FIG.

In the first embodiment of the present invention, unlike the second prior art, even though the widths W _a ″ and W _b ″ of the interactive optical waveguides 12a and 12b including the high refractive index regions 13a and 13b are equal, Refractive indexes are different from each other. Therefore, the equivalent refractive index (or propagation constant) of light propagating through the interaction optical waveguides 12a and 12b can be made different, and the coupling of light propagating through the two interaction optical waveguides 12a and 12b can be suppressed. it can. In this case, since it is not necessary to change the width of the interaction optical waveguides 12a and 12b, the distance Δ between the edge of the center conductor 4a and the center of the interaction optical waveguides 12a and 12b does not change. There is an advantage that the drive voltage does not increase as described in the above.

  Further, as described above, in the taper regions 6, 7, 8, 9, 10, and 11 shown in FIG. 15 shown as the second prior art, the width of the interactive optical waveguide changes, so that radiation loss occurs, and as a result, As a result, the insertion loss and the extinction ratio of the optical modulator are deteriorated. On the other hand, since such a tapered region does not exist in the first embodiment of the present invention, radiation loss caused by the tapered region does not occur. Furthermore, in the present invention, the high refractive index regions 13a and 13b are formed by thermal diffusion after additional deposition of Ti. Compared with the tapered regions 6, 7, 8, 9, 10, and 11 in the second prior art shown in FIG. 15, since the change in the refractive index formed by thermal diffusion is gradual, radiation loss hardly occurs. Therefore, this embodiment can be said to be an advantageous structure from the viewpoint of insertion loss and extinction ratio.

  Now think about chirping. The alpha parameter (or α parameter) representing the degree of chirping can be expressed as in equation (1) using the phase φ and intensity (amplitude) E of the optical signal pulse output from the optical modulator ( Non-patent document 1).

α = [dφ / dt] / [(1 / E) (dE / dt)] (1)
Thus, the α parameter can be expressed using the phase change amount and the intensity change amount of the output optical signal pulse.

  More specifically, the α parameter can be expressed by equation (2) obtained by developing equation (1).

α = (Γ ₁ −Γ ₂ ) / (Γ ₁ + Γ ₂ ) (2)
Γ ₁ ; Efficiency expressed by overlap integral normalized by 1 between the electric signal (amplitude) and the light (power) propagating through the interaction optical waveguide 12a (including 13a) Γ ₂ ; Interaction with the electric signal (amplitude) In the first region of efficiency length L ₁ indicated by the overlap integral normalized by 1 with the light (power) propagating through the optical waveguide 12b (including 13b), the high refractive index region 13a has an opposing interaction. The refractive index is higher than that of the optical waveguide 12b. On the other hand, in the second region of length L _2, the high refractive index region 13b has a higher refractive index than the interaction optical waveguides 12a opposed to each other. In general, since the spot size of light propagating through the regions 13a and 13b having a high refractive index is small, the overlap integral between the amplitude of the high-frequency electric signal and the light power in the region is an interactive optical waveguide having a low refractive index facing each other. It becomes larger than the overlap integral with the light propagating through 12b and 12a. Thus, greater than gamma ₁ are gamma ₂ in the first region, and greater than ₁ gamma ₂ is gamma in the second region.

As a result, the first embodiment finite length L ₁ of the first region in shown in FIG. 2, when the length L ₂ of the second region is set to 0, the interaction optical waveguides 12a, 12b between the Thus, even if the driving voltage can be reduced, the values of Γ ₁ and Γ ₂ described above are different, and chirping is expected to occur.

In FIG. 2, it is considered that the length L ₁ of the first region and the length L ₂ of the second region are finite lengths to reduce or eliminate this chirping. In a state where a high frequency electric signal is applied to the traveling wave electrode 4 composed of the center conductor 4a and the ground conductors 4b and 4c, the frequency f of the high frequency electrical signal of the traveling wave electrode 4 composed of the center conductor 4a and the ground conductors 4b and 4c. the microwave propagation loss and beta _m (f) in, in the center conductor 4a, a first region of length L _1, in the second region of length L _2, the interaction optical guide comprising a high refractive index regions 13a, 13b The efficiency of the interaction between the light propagating through the waveguides 12a and 12b and the electric signal is expressed as I ₁ (f) and I ₂ (f), respectively.

If the propagation direction of the optical and electrical signals is z, the efficiency of each interaction I ₁ (f), I ₂ (f) depends on the frequency f, and can be described by equations (3) and (4).

I ₁ (f) = ∫ ₀ ^L1 exp (−β _m (f) · z) dz
_{= (1-exp (-β m} (f) · L 1)) / β m (f) ... (3)
I ₂ (f) = ∫ _L1 ^L2 exp (−β _m (f) · z) dz
_{= Exp (-β m (f)} · L 1) · (1-exp (-β m (f) · L 2)) / β m (f)
…(Four)
Therefore, the conditions I ₁ (f) = I ₂ (f) (5) in which the efficiency of the interaction I ₁ (f) and I ₂ (f) are equal to each other.
The length L ₁ of the first area that satisfies, by setting the length L ₂ of the second region can chirping zero at arbitrarily designated frequency f. That is, the α parameter is zero when the equation (5) is satisfied.

From the length L ₁ of the first region and the length L ₂ of the second region that satisfy the formula (5), the interaction efficiency I ₁ (f) in the length L ₁ of the first region, the length of the second region The interaction efficiency I ₂ (f) in L ₂ is obtained, and further, the frequency characteristic of the α parameter at each frequency f of the high-frequency electric signal is obtained using the above-described equation (2).

  FIG. 5A and FIG. 5B show the frequency characteristics of the α parameter obtained. FIG. 5A shows a frequency characteristic in which the α parameter changes in sign from minus (−) to plus (+) as the frequency f of the applied high-frequency electrical signal increases, and FIG. Shows the frequency characteristic of the α parameter whose sign changes from plus (+) to minus (−) as the frequency f of the high-frequency electrical signal increases.

As described above, the length L ₁ of the first region is followed so that the sign of the α parameter indicating the chirping amount is switched in the low frequency region near DC and the high frequency region at the frequency f of the applied high frequency electrical signal. and set the length L ₂ of the second region. In order to realize this, L ₁ / L ₂ = 0.85. That is, it is necessary to set slightly longer than the length L ₁ of the length L ₂ of the second region the first region. However, since this value may vary depending on the structure of the electrode, it is needless to say the length of the first region L ₁ and the ratio of the length L ₂ of the second region is not limited to this.

  Note that (5) can be satisfied even when the α parameter is always positive or always negative and is zero only at a certain frequency, but it is important for the optical signal pulse emitted from the optical modulator. What is more effective is that the α parameter is set to zero on average within the required predetermined frequency band.

(Non-Patent Document 1) Nadege Courjal et al “Modeling and Optimization of Low Chirp LiNbO3 Mach-Zehnder Modulators With an Inverted Ferroelectric Domain Section“ Journal of Lightwave Technology vol.22 No.5 May 2004
Furthermore, consider improving the chirping characteristics of the optical modulator of the present invention described above. In other words, the chirping of the emitted optical signal pulse is considerably reduced in the configuration of the optical modulator described above, but the α parameter is zero on average within a predetermined frequency band necessary for the transmitted optical signal pulse in the optical communication system. Is more strongly demanded.

As shown in FIGS. 6 (a) and 6 (b), the maximum frequency in a predetermined frequency band required for the optical signal pulse is set to f _max, and as shown in FIG. 6 (a), the frequency f increases. Let us consider a case where the sign of the α parameter changes from minus (−) to plus (+). Of the frequency characteristics of the α parameter, the α parameter from the vicinity of DC to the frequency f ₀ where the α parameter becomes zero is α ₁ (f), and the α parameter from the frequency f ₀ to the maximum frequency f _max is α ₂ (f )
∫ ₀ ^f0 α ₁ (f) df = ∫ _f0 ^fmax α ₂ (f) df (6)
The length L ₁ of the first region and the length L ₂ of the second region are set so that This corresponds to making the area S ₁ and the area S ₂ equal in FIG.

Further, as shown in FIG. 6B, consider a case where the sign of the α parameter changes from plus (+) to minus (−) as the frequency f increases. If the α parameter from the vicinity of DC to the frequency f ₀ where the α parameter becomes zero is α ₃ (f), and the α parameter from the frequency f ₀ to the maximum frequency f _max is α ₄ (f),
∫ ₀ ^f0 α ₃ (f) df = ∫ _f0 ^fmax α ₄ (f) df (7)
The length L ₁ of the first region and the length L ₂ of the second region are set so that This corresponds to making the area S ₃ and the area S ₄ equal in FIG. 6B.

In order to realize this, L ₁ / L ₂ = 0.76 was a preferable value. However, since this value varies depending on the electrode structure, it is needless to say that this is not the case. Further, the maximum frequency included in the optical signal pulse differs depending on the optical transmission method, and is not limited to the embodiment illustrated as an example.

5 (a) and 5 (b), the frequency at which the α parameter is made zero, and the optical modulators shown in FIGS. 6 (a) and 6 (b) (6), ( Naturally, it is different from the frequency f ₀ satisfying the equation (7).

  Further, when an electrical signal is Fourier-transformed and the electrical signal has many components in a specific frequency region, integration may be performed as follows using a weight W (f) representing the frequency region. .

∫ ₀ ^f0 α ₁ (f) W (f) df = ∫ _f0 ^fmax α ₂ (f) W (f) df
… (8)
[Second Embodiment]
FIG. 7 shows a second embodiment of the present invention. Here, only the optical waveguide 14 among the components of the optical modulator is shown. Two regions a high refractive index in the interaction optical waveguides 14a in the present embodiment, i.e. the first region and the third region of length L ₅ of the length L ₃ provided, similarly refractive index in the interaction optical waveguide 14b It is provided with the second region of length L ₄ as a high region.

High frequency electric signal is the center electrode 4a and the ground electrodes 4b (not shown), since the attenuation while propagating a traveling wave electrode 4 made of 4c, in the first region of the length L ₃ is the strongest intensity, the length L ₅ In the third region, the intensity is the weakest. Further, in the second region of length L ₄ whose intensity is intermediate the first region and the third region. Therefore, the chirping of light can be made sufficiently small as a whole. Needless to say, this approach may provide more than three high refractive index regions in the interaction optical waveguides 14a and 14b.

  The present invention is not limited to the above-described embodiments.

[About each embodiment]
In the above description, the CPW electrode has been described as an example of the traveling wave electrode. However, it goes without saying that various traveling wave electrodes such as an asymmetric coplanar strip (ACPS) and a symmetric coplanar strip (CPS), or a lumped constant electrode may be used. .

  In the above embodiment, the crystal orientation of x-cut, y-cut or z-cut, that is, the x-axis, y-axis or z-axis of the crystal is perpendicular to the substrate surface (cut plane). The surface orientation in each of the above-described embodiments may be a main surface orientation, and other surface orientations may be mixed as a surface orientation subordinate to this, and not only an LN substrate but also a lithium tantalum. It goes without saying that other substrates such as rates and semiconductors may be used.

  As described above, the optical modulator according to the present invention is useful as an optical modulator having high speed, a small alpha parameter, a large extinction ratio, and a small driving voltage and DC bias voltage.

Sectional drawing of the optical modulator of the 1st Embodiment of this invention The top view of the optical waveguide of the 1st Embodiment of this invention The figure which shows the refractive index distribution of the optical waveguide of the 1st Embodiment of this invention The figure which shows the relationship between light coupling | bonding degree and | (DELTA) n |. The figure which shows the frequency characteristic of alpha parameter of light The figure which shows the frequency characteristic of alpha parameter of light The top view of the optical waveguide of the 2nd Embodiment of this invention Perspective view of the first prior art Sectional drawing in the A-A 'line of 1st prior art Top view of first prior art optical waveguide The figure explaining operation | movement of 1st prior art Diagram showing the relationship between Vπ · L and Δ The figure which shows the relationship between the coupling _| bonding degree of light and _Gwg Sectional view of the second prior art Top view of second prior art optical waveguide

Explanation of symbols

1: x-cut LN substrate (substrate)
2: SiO ₂ buffer layer (buffer layer)
3, 5, 12, 14: Optical waveguide 3a, 3b, 5a, 5b, 12a, 12b, 14a, 14b: Interaction optical waveguide 4: Traveling wave electrode 4a: Center conductor 4b, 4c: Ground conductor 6, 7, 8 9, 10, 11: Tapered region 13a, 13b, 15a, 15b, 15c: High refractive index region

Claims (7)

A substrate having an electro-optic effect, an optical waveguide for guiding light formed on the substrate, and a high-frequency electric signal formed on one surface side of the substrate for modulating the light A traveling wave electrode comprising a central conductor and a ground conductor for a high frequency electrical signal, and the optical waveguide has a plurality of components for modulating the phase of the light by applying the high frequency electrical signal to the traveling wave electrode. In an optical modulator comprising a Mach-Zehnder optical waveguide having an interactive optical waveguide,
Each of the plurality of interaction optical waveguides has at least one region in the longitudinal direction having a region where the refractive indexes of the plurality of interaction optical waveguides are different from each other in a cross section perpendicular to the longitudinal direction of the optical waveguide. An optical modulator comprising the optical modulator.   The region is provided in each of the interaction optical waveguides so that the magnitude relationship of the refractive index in the longitudinal direction is switched between the interaction optical waveguides facing each other. The light modulator described.   The refractive index magnitude relationship between the two interacting optical waveguides facing each other is switched in the longitudinal direction of the interactive optical waveguide so that the chirping of the light modulated by the high-frequency electrical signal becomes substantially zero. 3. The optical modulator according to claim 2, wherein the length of the region is set. The magnitude relationship between the refractive indexes of the two interacting optical waveguides facing each other is changed with the lengths of L ₁ and L ₂ in the longitudinal direction of the interactive optical waveguide, and L ₁ and L ₂ are L ₁ <L The optical modulator according to claim _{2, wherein} the relationship ₂ is satisfied. The magnitude relationship between the refractive indexes of the two interacting optical waveguides facing each other is changed with the lengths L ₁ , L ₂ , and L ₃ in the longitudinal direction of the interactive optical waveguide, and the L ₁ , L ₂ , L ₃ is an optical modulator according to claim 2 or 3, characterized in that substantially satisfy a relation of _{L 1 = L 3 = L 2} /2.   The sign of the alpha parameter indicating the chirping magnitude of the optical signal pulse emitted from the interaction optical waveguide within a predetermined modulation frequency band based on the high-frequency electrical signal is changed from plus to minus with the modulation frequency change. The length of the region where the magnitude relationship of the refractive index in the two interacting optical waveguides facing each other is changed in the longitudinal direction of the optical waveguide is set so as to change from negative to positive. The optical modulator according to claim 2.   The magnitude relationship between the refractive indexes of the two interacting optical waveguides facing each other is such that the integral value for the frequency of the alpha parameter in a predetermined frequency band based on the high-frequency electrical signal is substantially zero. 5. The optical modulator according to claim 2, wherein a length of the region that is switched in a longitudinal direction of the optical waveguide is set.

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